A helical wrapping technique that is conventionally used only by simple machines operating at a low rate of throughput, stretches a film directly on a palletized load in the form of a rectangular parallelepiped. The film is taken from a vertical axis reel fitted with a brake. One end of the film is initially fixed to the load, after which the load is caused to rotate about a vertical axis. The reel is braked in such a manner as to cause the film to be stretched by the desired amount.
Unfortunately, since the load is not circularly cylindrical, but generally forms a rectangular parallelepiped, the radial distance to be covered with the film varies with respect to the particular portion or region of the load being wrapped, so that rotating the load at a constant angular velocity while applying a constant braking force causes the film to be elongated in a non-uniform manner. Also, immediately after stretching, the stretchable film is subject to relaxation which leads, over a period of approximately 48 hours, to a drop in its resilient return force that is substantially equal to 50%. Thus, with a machine of this type, there is a limit set by the stretching forces that can be withstood by the load, whereas the resilient forces that subsequently provide cohesion to the load during handling and transport are significantly lower. These factors have led to the above type of machine being abandoned in favor of wrapping machines including a pre-stretching device comprising a plurality of motor-driven rollers operating at different peripheral speeds. At the outlet from the rollers of the pre-stretching device, the film is wrapped without tension, or preferably under low tension, on the load to be wrapped, as explained with reference to FIG. 1.
In patent application No. 92/10254 (published under No. FR 2 695 102), the Applicant describes a method of wrapping a load in a previously-stretched film, with the film being put into place only after a period has elapsed to allow the film to relax while still under tension. As explained below with reference to FIG. 2, that patent application indicates that it is thus possible to achieve an improvement with respect to residual elongation, after the film has relaxed, thereby making it possible to consume less film.
In patent application No. 92/10254 (published under No. FR 2 695 102), the Applicant describes the optimum way of using a previously-stretched film with relaxation under a high level of tension.
However, films relaxed under a high level of tension are not suitable for all types of wrapping. Firstly, they exert a large constriction force on the film reel, which force can, in some cases, be great enough to destroy such a reel. It is thus necessary to use reels that are very strong in compression, for example, those made of steel. Such reels are expensive and, in most cases, must be reused. The return of empty reels gives rise to non-negligible costs. Also, because of its elastic memory, the film placed around a load exerts a constriction force which has a value that is substantially equal to half the value of the tension under which the film was relaxed. Thus, some loads are too fragile or too compressible to withstand the constriction forces exerted by a film that has been relaxed under a high level of tension. Further, films relaxed under a high level of tension are rather stiff, are not very resilient, and they are often fragile. In some cases, such films present a high risk of being punctured, in particular at the corners of the pallet.
WO 89/06594 describes a device for producing thin films that takes best advantage of the production capacities of an extruder. A relatively thick film is initially extruded and its area can be increased by stretching.
In that document, it is desired to release the force in the film as much as possible prior to relaxation in order to wind the film around a load with a force that is as low as possible.